Is there a (convenient) way in WER to change a player's DCI number after the event had begun?
It seems to let you change names/countries, but not numbers.

There is no convenient way that I am aware of.

As with all these problems though, you can save the tournament, edit the save-file with the text editor of your choice (I prefer Notepad++), save the changes and import the event, overwriting the old one. Works like a charm on any event, really. Just make sure to replace all instances of the changed DCI# or live in fear of copious amounts of error messages. ;)

I believe the only supported method is contacting your WPN Rep prior to reporting the event.

I would strongly advise against manually editing of WER files as doing so could easily break your tournament or even your WER installation.

I agree with Mark regarding the fact that bad edits can actually make you unable to continue the tournament.
I would be very surprised if you manage to somehow break your WER installation by editing an event save though. That would take an unusual amout of… er, “talent”.

As a general guideline, if you feel comfortable editing *.ini files on your computer (for games, for example) then you should easily be able to find your way around in exported tournament files. It's really dead easy.

If you don't - do as Mark said and contact your WPN Rep.

Solution that worked for me a few time I had this problem is:
1. add to event new player with correct DCI number
2. repair all matches of player with wrong number, pair new player with his opponent and give correct result to match, and match loss to “wrong” player
3. after that: drop player with wrong number from tournament. at that moment he shouldn't have any rounds played, so he should disappear from program entirely.

But it was a year or more ago, it is possible that WER behaves differently now.